In vehicles, a catalyst body is provided midway along an exhaust passage in an exhaust system in order to purify exhaust emissions which are discharged from an internal combustion engine. Among such constructions having the catalyst body placed in the exhaust system, there is one type of structure which is provided with a catalyst deterioration-determining device.
Some of the catalyst deterioration-determining devices are constructed to provide what is called dual feedback control. More specifically, this type of catalyst deterioration-determining device has front and rear oxygen sensors disposed in an exhaust passage of an internal combustion engine respectively on upstream and downstream sides of a catalyst body. The catalyst body is placed midway along the exhaust passage. The front oxygen sensor acts as a first exhaust sensor, while the rear oxygen sensor serves as a second exhaust sensor. The catalyst deterioration-determining device executes feedback control of an air-fuel ratio in accordance with respective detection signals from the front and rear oxygen sensors. At the same time, this device determines the deterioration of the catalyst body when predetermined catalyst deterioration-determining conditions are fulfilled. To be specific, the catalyst deterioration-determining device performs first feedback control of the air-fuel ratio to a target value on the basis of a first feedback control value. The first feedback control value is calculated from a first detection signal that is generated from the front oxygen sensor. The device further executes second feedback control in which a degraded state of the catalyst body is judged by a second feedback control value, thereby correcting the first feedback control value. The second feedback control value is derived from a second detection signal that is generated from the rear oxygen sensor.
The preceding devices for determining the catalyst deterioration are disclosed, e.g., in published Japanese Patent Applications Laid-Open Nos. 4-109045 and 4-116239.
According to above-mentioned Publication No. 4-109045, the catalyst deterioration-determining device includes: a purifying means located in an exhaust pipe of an internal combustion engine; air-fuel ratio-detecting means disposed individually on upstream and downstream sides of the purifying means; a fuel-supplying means for supplying fuel to cylinders of the internal combustion engine; and, an electronic control means for controlling the fuel-supplying means upon receiving respective detection signals from the air-fuel ratio-detecting means. As a result of this construction, an air-fuel ratio of a mixture to be supplied to the cylinders is controlled on the basis of the detection signal from one of the air-fuel ratio-detecting means, which is located on the upstream side of the purifying means. In addition, the detection signal from the other of the air-fuel ratio-detecting means, which is positioned on the downstream side of the purifying means, responds differently to different target values of the air-fuel ratio. Accordingly, the deterioration of the purifying means is monitored by such different responses of the detection signal. As a result, determination with increased reliability can be made in shorter time as to whether the catalyst has been degraded.
According to the device of aforementioned Publication No. 4-116239, air-fuel ratio sensors are disposed individually on upstream and downstream sides of a catalytic converter, and comparison is made between respective output signals from the air-fuel ratio sensors, thereby determining the catalyst deterioration. At this time, however, if it is found that learning correction using the aforementioned downstream side sensor has insufficiently been renewed, this catalyst deterioration-determining device prohibits further diagnosis of the learning correction. This feature prevents both reduced accuracy of diagnosis and inconsistent criteria of judgment, each of which would occur as a result of an actual air-fuel ratio itself being deviated.
In the preceding devices for determining the catalyst deterioration, there is nothing to materially diminish the function of the catalyst body so far as vehicles which are in normal use.
However, the function of the catalyst body is dramatically reduced when, e.g., a vehicle user operates a vehicle with lead-containing fuel, or when a vehicle is traveled in a state of a high-tension cord being pulled out of position as a result of unexpected causes. The reduced function of the catalyst body reduces exhaust emission-purifying efficiency. This causes an inconvenience in that there is an increased possibility of exhaust steam being released in the air, which results in environmental aggravation.
In addition, a temperature state of the catalyst body is detected by a temperature sensor, and a warm-up state (an activated temperature state) and an oxygen storage state (an oxygen-retaining state) of the catalyst body is judged by the detected temperature state which is indicated by a detection signal from the temperature sensor. In this way, this step requires the temperature sensor, which creates another inconvenience of a complicated, high cost structure.
Furthermore, with respect to execution conditions of determining (or, monitoring) the deterioration of the catalyst body, a problem occurs with a catalyst deterioration-determining device adapted to simulate and determine the deterioration of the catalyst body in accordance with the oxygen storage ability of the catalyst body. More specifically, there are cases where an oxygen storage capacity of the catalyst body is reduced, depending on the warm-up state (the activated temperature state) of the catalyst body. In particular, the oxygen storage capacity is decreased when the catalyst body is cooled down by being exposed to wind at low temperatures during vehicle traveling. The reduced oxygen storage capacity also occurs during long regimes of low load traveling, idle operation, or high load traveling. Thereafter, the oxygen storage capacity of the catalyst body remains diminished during lapse of some seconds up to some minutes. As a result of the reduced oxygen storage capacity, the catalyst body is determined to be abnormal in spite of being normal, thereby involving misjudgment on the deterioration of the catalyst body. This causes an inconvenience of requiring meaningless replacement or repair of parts.
In order to obviate the aforesaid inconveniences, a first aspect of the present invention provides a catalyst deterioration-determining device for an internal combustion engine, having first and second exhaust sensors disposed in an exhaust passage of the internal combustion engine respectively on upstream and downstream sides of a catalyst body, the catalyst body being placed midway along the exhaust passage, the catalyst deterioration-determining device effecting feedback control of an air-fuel ratio in response to respective detection signals from the first and second exhaust sensors, and simultaneously determining the deterioration of the catalyst body when predetermined catalyst deterioration-determining conditions are fulfilled, the improvement comprising: a control means which judges, on the basis of a warm-up counter used for the catalyst body, whether or not the catalyst body is in a predetermined warm-up state, the warm-up counter reckoning in relation to any engine load of the internal combustion engine, wherein assuming that the catalyst body is found to be in the predetermined warm-up state according to the above judgment, the control means determines whether or not deterioration-determining conditions for the catalyst body have been fulfilled.
A second aspect of the present invention provides a catalyst deterioration-determining device for an internal combustion engine according to the first aspect, wherein after the internal combustion engine is started, the control means drives the warm-up counter to state reckoning in relation to any engine load of the internal combustion engine, whereby the control means judges that the catalyst body is in a predetermined warm-up state when counts registered by the warm-up counter fall within a range of catalyst deterioration-determining execution, and wherein the control means determines the degradation of the catalyst body in the event that the catalyst body is known by the above judgment to be in the predetermined warm-up state, and that other catalyst deterioration-determining conditions are all satisfied.
According to the structure incorporating the present invention, there is provided the control means which determines, on the basis of the warm-up counter, whether or not the catalyst body is in a predetermined warm-up state. The warm-up counter is used for the catalyst body, and counts in relation to any engine load of the internal combustion engine. Assuming the catalyst body is found to be in the predetermined warm-up state according to the above determination, the control means judges whether or not deterioration-determined conditions for the catalyst body have been fulfilled. As a result, a warm-up state and an oxygen storage state of the catalyst body can be simulated and determined without the use of a temperature sensor. Accordingly, there is no need for the temperature sensor, and a simple, low cost structure is provided.
In addition, since a temperature state of the catalyst body known from the temperature sensor is ignored from the deterioration-determining conditions for the catalyst body, accurate determination can be made to the degradation of the catalyst body. Consequently, improvements are achievable in both the reliability of vehicles and the accuracy of judgment on the deterioration of the catalyst body.
Furthermore, when the temperature of the catalyst body under the influence of low temperatures and/or a strong wind is lowered to a level incapable of determining the deterioration of the catalyst body, the catalyst deterioration-determining conditions are identified as not being fulfilled in spite of the catalyst body being in normal operation. Therefore, determination is stopped from being made to the deterioration of the catalyst body. As a result, there is no likelihood that the catalyst body in a normal state might be judged as abnormal. This means that a misjudgment is not made as to the degradation of the catalyst body, thereby obviating the need for needless replacement or repair of parts.